The ongoing retreat of glaciers at southern sub-polar latitudes is particularly rapid and widespr... more The ongoing retreat of glaciers at southern sub-polar latitudes is particularly rapid and widespread. Akin to northern sub-polar latitudes, this retreat is generally assumed to be linked to warming. However, no long-term and well-constrained glacier modeling has ever been performed to confirm this hypothesis. Here, we model the Cook Ice Cap mass balance on the Kerguelen Islands (Southern Indian Ocean, 49°S) since the 1850s. We show that glacier wastage during the 2000s in the Kerguelen was among the most dramatic on Earth. We attribute 77% of the increasingly negative mass balance since the 1960s to atmospheric drying associated with a poleward shift of the mid-latitude storm track. Because precipitation modeling is very challenging for the current generation of climate models over the study area, models incorrectly simulate the climate drivers behind the recent glacier wastage in the Kerguelen. This suggests that future glacier wastage projections should be considered cautiously where changes in atmospheric circulation are expected. Remote sensing data in the southern mid-latitudes have documented the occurrence of rapid and extensive gla-cial retreat 1–5. However, full glaciological modeling is rarely used to identify the drivers of the ice wastage in these regions. Instead, interpretations are largely qualitative, based on simple comparisons with regional climatic trends. Recent modelling studies of the Patagonian Icefields are one exception 6,7 , but the studies only address changes in the surface mass balance since the mid-1970s and climatic interpretations were complicated by the prevalence of calving glaciers. The lack of full glaciological modeling studies has led to the main conclusion that, akin to northern sub-polar latitudes 8–11 , recent ice wastage at southern mid-latitudes can mainly be attributed to warming 1–5 , whereas no consensus has been reached on the role of possible changes in precipitation 1,5–7,12. Because long-term, continuous, and reliable field measurements on and close to glaciers and ice caps are particularly scarce in the southern hemisphere mid-latitudes, conclusions on climatic trends and impacts are particularly uncertain. Here we advanced understanding of the climate drivers of glacial wastage at southern mid-latitudes by analyzing variations in the Cook Ice Cap (CIC, covering 410 km 2 in 2001) multi-decadal mass balance in the Kerguelen Islands in the southern Indian Ocean (49°S, 69°E, Fig. 1a). This ice cap is mainly composed of land-terminating glaciers and variations in its mass are more directly linked with climate variations than other ice caps with calving termini at the same latitudes. The continuous long term observations made since 1951 at Kerguelen (Port-aux-Français (PAF) station), plus dated geomorphologic evidence since the Little Ice Age (LIA) 3,13,14 , and glaciological observations since the 1970s, comprise an exceptionally long glacio-meteorological dataset. We used this unique set of long-term climate and glaciological observations in the southern sub-polar latitudes, the extended historical reconstruction of sea surface temperatures (SST) from ERSST.v2 15 available since 1854, and old 3 and new remote sensing mass balance measurements to constrain a mass balance model run from 1850 to 2011 to analyze the climatic drivers of the post-1960s retreat.
The Alps are highly impacted by debris flows that cause major problems for companies and transpor... more The Alps are highly impacted by debris flows that cause major problems for companies and transport networks located in the valley bottoms. One such event occurred in the Rif Blanc catchment and affected the road network in the French Alps, as well as adjacent areas across the Italian border, for several days in June 2012. This article presents two independent approaches to vulnerability assessment. Based on investigations conducted during a survey of local authorities following the event, we compared theoretical risk management and real crisis management in terms of decision making and modes of intervention. Functional vulnerability and territorial consequences were analyzed using a best travel time model of accessibility. We show that a bottom-up approach is practiced in case of actual management planning with a central coordination of general council. Conversely theoretical crisis management shows prefect as the key actor supported by several other state institutions. Our analysis also revealed that a debris flow event with a local impact on the road network has territorial consequences at a regional scale. This study contributes to the discussion about how to minimize the vulnerability of alpine transport networks prone to debris flows. Our results could serve as a decision support tool for public authorities.
The aim of this paper is firstly to characterise the debris flow frequency since the beginning of... more The aim of this paper is firstly to characterise the debris flow frequency since the beginning of the 20 th century in 6 valleys located on the eastern part of the Massif des Ecrins (French Alps). This analysis is based on the dating of 142 debris flow deposits by lichenometry, dendrochronology, aerial photographs analysis from 1952 and old documents. Results show that it is very difficult to compare the two periods of 1900-1950 and 1950-2000 because the number of debris flows over the first half of the century is probably underestimated. Over the second period , the frequency of debris flow has decreased significantly for the 200-399 m class lengths. We have also observed a decrease in the number of debris flows between 1950-1975 and 1975-2000. We have made two logistic regression models in order to determine an frequency probability according to geomorphological and climatic parameters. The first model shows that the frequency probability depends on the debris flow triggering zone altitude and the surface of the rock face, while the second model shows that the frequency probability depends on rainy events greater than 30 mm/day and also on the cumulated number of freezing days.
To assess the impacts of future climatic change, we need a better understanding of the relationsh... more To assess the impacts of future climatic change, we need a better understanding of the relationships between debris flows dynamic and the climate. The subject of this paper is the variability in the response of debris flows to climatic change in recent decades in the Massif des Ecrins (French Alps). The climatic trend observed in the French Alps was characterized by analyzing data on extreme summer rainfall events recorded daily at nine stations located all over the Massif des Ecrins since 1960. According to the generalized Pareto law (GPD) our results showed that extreme summer rainfalls have increased significantly in the Massif des Ecrins since the 1980s. Statistical tests showed a significant increase in average annual air temperature. The response of 647 hill slope debris flows to this climatic change was investigated. Different types of hill slope debris flows were analyzed as a function of their lithology or the nature of the accumulated debris. A number of logistic regression based models were developed to characterize the relationship between climate and the occurrence of debris flows in a specific geomorphological context. Analysis of frequency and return period over the last two decades showed different patterns depending on the type of debris flow concerned. Hence, the response of hill slope debris flows to climatic change is not the same everywhere in the Massif des Ecrins.
In this paper we investigate the impacts of future climatic change on the occurrence of debris fl... more In this paper we investigate the impacts of future climatic change on the occurrence of debris flows in the Massif des Ecrins (French Alps). Two distinct aspects are discussed: the impact of future climatic change on the evolution of the process, and changes in the spatial distribution. Three climate simulations are presented for the current period (1970–1999) and for future periods (2070–2099) using GCM ARPEGE CLIMAT model developed by Météo-France. Simulated data are then statistically downscaled to obtain a higher spatial resolution. In the first step, we compare occurrence probabilities in the current period and in the next century. In the second step, we estimate which zones would be affected by the process in the future at the scale of the Massif des Ecrins. For the current period, the best model was obtained between debris flows and the number of days between June 15th and October 15th with more than 20 mm calculated either from observed meteorological or simulated data. Results of the ARPEGE model considering the A2 hypothesis (IPCC 2007) showed that the most significant climatic trends for the end of the century will be a decrease in intense rainy events and an increase in temperature. These trends are expected to reduce the occurrence of hill slope debris flows in the Massif des Ecrins. From a spatial point of view, the increase in temperature should
Vincent, some of this paper is in British English and some in American, which changes the spellin... more Vincent, some of this paper is in British English and some in American, which changes the spelling modelling/modeling etc etc and you should have the same spelling throughout Abstract Reconstructing climate over the last millennium is one major target of climate research. The extent of climate variability over recent centuries sets the scale on which ongoing climate changes can be measured. The quality of reconstructions of millennial temperatures in the northern hemisphere has greatly improved over the last decade (IPCC, 2007). Nevertheless there is still a lack of precisely dated, high-quality climate reconstructions in the tropics, a lack which is particularly disturbing since this region is a major climate engine for the global water cycle. In recent years, a significant effort has been made to improve our knowledge on climatic variability in the tropical Andes by developing new proxy records, new transfer functions and better time resolution of available proxy records. Hence the aim of this paper is to review the different proxies with multi-decadal to annual resolution available in the tropical Andes (Bolivia, Peru, Ecuador) such as glacial moraines, pollens, isotopic variations in ice core records, and tree rings. For each proxy, we focus on recent major methodological progress and on new results revealing climatic shifts over the last millennium.
A total of 1500 spacing distances of sorted stripes were measured on 30 alpine sites in the Boliv... more A total of 1500 spacing distances of sorted stripes were measured on 30 alpine sites in the Bolivian Andes with distinct slope angles, elevations and aspects. Additionally, the geometric evolution of stripes was systematically analysed along a slope with varying inclinations and morphology. We dealt with mainly small-sized stripes (<40 cm). The scatter observed in the distribution of spacing distances suggests that the factors controlling the geometry of sorted stripes are complex. Among them, we point out the role of slope inclination and coarse sediment supply. They imply that stripe geometry does not depend exclusively on processes operating in soils such as frost heaving and sorting, but also on the dynamics of the flux of coarse materials along the sloping surface. This analysis questions models which assume that sorted stripes are organized according to a strict periodicity whose origin depends on specific physical processes. Particularly, it appears that recent convective models cannot explain the characteristics and the evolution of patterned slopes in this environment. R ´ ESUMÉ On a mesuré 1500 largeurs de stries réparties sur 30 sites se différenciant par la pente, l'altitude et l'exposition. Par ailleurs, on a analysé systématiquement l'´ evolution des stries le long d'un versant de pente et de morphologie variables. On confirme que dans ce type de milieu o` u dominent les cycles de gels-dégels journaliers les stries sont en majorité de petite dimension (<40 cm), mais la dispersion de la distribution suggère que les facteurs qui déterminent l'espacement des stries sont complexes. Parmi eux se détachent l'inclinaison de la pente et la morphologie du haut du versant, laquelle intervient par la quantité de produits clastiques parvenant au sol strié. Cela implique que la géométrie des sols striés ne dépende pas seulement des mécanismes du gel dans le sol, comme le gonflement et le tri qui lui est associé, mais aussi de la dynamique du flux de matériel grossier tout au long du versant. Cetté etude remet en question les modèles qui partent de l'hypothèse que les sols striés sont organisés selon une stricte périodicité, dont l'origine est attribuéè a un processus physique particulier. Il apparaˆıt notamment que les modèles construitsà partir de la convection ne parviennent pasà rendre compte des caractères et desévolutions observés sur les sols striés dans ce type de milieu.
SUMMARY Currently, there is a tremendous scientific research effort in the area of climate change... more SUMMARY Currently, there is a tremendous scientific research effort in the area of climate change. In this paper, our motivation is to improve the understanding of historical climatic events such as the Little Ice Age (LIA), a period of relatively cold weather around 1450–1850 AD. Although the LIA is well documented in Europe, its extent and timing are not known in areas of the globe where climatological records were not kept during this period. To study the climate, which predates historical records, proxy climate records must be used. A proxy record for the timing of climatic cooling events are the ages of the moraines left behind by glacial advances. Unfortunately, to determine the ages of these moraines in alpine environments there is little material available but lichens. Hence, lichenometry was developed to determine the ages of glacial landforms by using lichen measurements. To our knowledge, this article provides the first attempt at deriving a comprehensive statistical model for lichenometry. Our model foundation is based on extreme value theory because only the largest lichens are measured in lichenometry studies. This application is novel to extreme value theory because the quantities of interest (the ages of climatic events) are not the measured quantities (lichen diameters), i.e., it is a inverse problem. We model the lichen measurements with the generalized extreme value (GEV) distribution, upon which a Bayesian hierarchical model is built. The hierarchical model enables estimation of the hidden covariate ages of the moraines. The model also allows for pooling of data from different locations and evaluation of spatial differences in lichen growth. Parameter inference is obtained using a straightforward Markov Chain Monte Carlo method. Our procedure is applied to data gathered from the Cordillera Real region in Bolivia.
In this paper, we discuss the advantages and drawbacks of the most classical approaches used in l... more In this paper, we discuss the advantages and drawbacks of the most classical approaches used in lichenometry. In particular, we perform a detailed comparison among methods based on the statistical analysis of either the largest lichen diameters recorded on geomorphic features or the frequency of all lichens. To assess the performance of each method, a careful comparison design with well-defined criteria is proposed and applied to two distinct data sets. First, we study 350 tombstones. This represents an ideal test bed because tombstone dates are known and, therefore, the quality of the estimated lichen growth curve can be easily tested for the different techniques. Secondly, 37 moraines from two tropical glaciers are investigated. This analysis corresponds to our real case study. For both data sets, we apply our list of criteria that reflects precision, error measurements and their theoretical foundations when proposing estimated ages and their associated confidence intervals. From this comparison, it clearly appears that two methods, the mean of the n largest lichen diameters and the recent Bayesian method based on extreme value theory, offer the most reliable estimates of moraine and tombstones dates. Concerning the spread of the error, the latter approach provides the smallest uncertainty and it is the only one that takes advantage of the statistical nature of the observations by fitting an extreme value distribution to the largest diameters.
The Younger Dryas stadial, a cold event spanning 12,800 to 11,500 years ago, during the last degl... more The Younger Dryas stadial, a cold event spanning 12,800 to 11,500 years ago, during the last deglaciation, is thought to coincide with the last major glacial re-advance in the tropical Andes1. This interpretation relies mainly on cosmic-ray exposure dating of glacial deposits. Recent studies, however, have established new production rates2–4 for cosmogenic 10Be and 3He, whichmake it necessary to update all chronologies in this region1,5–15 and revise our understanding of cryospheric responses to climate variability. Here we present a new 10Be moraine chronology in Colombia showing that glaciers in the northern tropical Andes expanded to a larger extent during the Antarctic cold reversal (14,500 to 12,900 years ago) than during the YoungerDryas.On the basis of a homogenized chronology of all 10Be and 3He moraine ages across the tropical Andes, we show that this behaviour was common to the northernand southern tropical Andes. Transient simulations with a coupled global climate model suggest that the common glacier behaviour was the result of Atlanticmeridional overturning circulation variability superimposed on a deglacial increase in the atmospheric carbon dioxide concentration. During the Antarctic cold reversal, glaciers advanced primarily in response to cold sea surface temperatures over much of the Southern Hemisphere. During theYoungerDryas, however,northern tropicalAndes glaciers retreated owing to abrupt regional warming in response to reduced precipitation and land–surface feedbacks triggered by a weakened Atlantic meridional overturning circulation. Conversely, glacier retreat during the Younger Dryas in the southern tropical Andes occurred as a result of progressive warming, probably influenced by an increase in atmospheric carbon dioxide. Considered with evidence frommid-latitude Andean glaciers16, our results argue for a common glacier response to cold conditions in the Antarctic cold reversal exceeding that of the Younger Dryas.
Much work has been done to show that there is a relationship between the triggering of debris flo... more Much work has been done to show that there is a relationship between the triggering of debris flows and the recorded increase in temperatures or in the number of intense rainy events over the last few decades. The question addressed in this paper is that of the impact of these climate changes on the dynamics of debris flows since the 1950s. 319 debris flows in the Dévoluy and Ecrins massifs located in the French Alps, the triggering of which is independent of the current glacial retreat, have been analysed. In the Dévoluy a reduction in the number of debris flows was observed in the periods 1950–1975 and 1975–2000. In the Massif des Ecrins, we have observed a shift of the triggering debris flow zone toward higher elevations and a lack of significant variation in the number of debris flows. But in the Massif des Ecrins this global result masks two different trends depending on elevation. At low altitude (<2200 m) the number of debris flows and the frequency of debris flows less than 400 m in length have decreased significantly since the 1980s whereas no significant variation was observed at high altitude (>2200 m). At the same time, we have observed a significant increase in the annual and seasonal temperatures for these 20 last years combined with a significant reduction in the number of freezing days. A significant increase in summer rains higher than 30 mm/d has also been observed. In Dévoluy and at low altitude in the Massif des Ecrins, these variations can be explained by the decrease in the number of freezing days related to the increase in the temperatures, which implies a slower reconstitution of the volume of debris stored between two events. But at high altitude it is currently difficult to establish the link between the climatic change and the dynamics of the debris flows because very little is known about the two variables controlling the triggering of the debris flows, i.e., on the one hand intense precipitations and on the other hand the volume of rock debris.
The ongoing retreat of glaciers at southern sub-polar latitudes is particularly rapid and widespr... more The ongoing retreat of glaciers at southern sub-polar latitudes is particularly rapid and widespread. Akin to northern sub-polar latitudes, this retreat is generally assumed to be linked to warming. However, no long-term and well-constrained glacier modeling has ever been performed to confirm this hypothesis. Here, we model the Cook Ice Cap mass balance on the Kerguelen Islands (Southern Indian Ocean, 49°S) since the 1850s. We show that glacier wastage during the 2000s in the Kerguelen was among the most dramatic on Earth. We attribute 77% of the increasingly negative mass balance since the 1960s to atmospheric drying associated with a poleward shift of the mid-latitude storm track. Because precipitation modeling is very challenging for the current generation of climate models over the study area, models incorrectly simulate the climate drivers behind the recent glacier wastage in the Kerguelen. This suggests that future glacier wastage projections should be considered cautiously where changes in atmospheric circulation are expected. Remote sensing data in the southern mid-latitudes have documented the occurrence of rapid and extensive gla-cial retreat 1–5. However, full glaciological modeling is rarely used to identify the drivers of the ice wastage in these regions. Instead, interpretations are largely qualitative, based on simple comparisons with regional climatic trends. Recent modelling studies of the Patagonian Icefields are one exception 6,7 , but the studies only address changes in the surface mass balance since the mid-1970s and climatic interpretations were complicated by the prevalence of calving glaciers. The lack of full glaciological modeling studies has led to the main conclusion that, akin to northern sub-polar latitudes 8–11 , recent ice wastage at southern mid-latitudes can mainly be attributed to warming 1–5 , whereas no consensus has been reached on the role of possible changes in precipitation 1,5–7,12. Because long-term, continuous, and reliable field measurements on and close to glaciers and ice caps are particularly scarce in the southern hemisphere mid-latitudes, conclusions on climatic trends and impacts are particularly uncertain. Here we advanced understanding of the climate drivers of glacial wastage at southern mid-latitudes by analyzing variations in the Cook Ice Cap (CIC, covering 410 km 2 in 2001) multi-decadal mass balance in the Kerguelen Islands in the southern Indian Ocean (49°S, 69°E, Fig. 1a). This ice cap is mainly composed of land-terminating glaciers and variations in its mass are more directly linked with climate variations than other ice caps with calving termini at the same latitudes. The continuous long term observations made since 1951 at Kerguelen (Port-aux-Français (PAF) station), plus dated geomorphologic evidence since the Little Ice Age (LIA) 3,13,14 , and glaciological observations since the 1970s, comprise an exceptionally long glacio-meteorological dataset. We used this unique set of long-term climate and glaciological observations in the southern sub-polar latitudes, the extended historical reconstruction of sea surface temperatures (SST) from ERSST.v2 15 available since 1854, and old 3 and new remote sensing mass balance measurements to constrain a mass balance model run from 1850 to 2011 to analyze the climatic drivers of the post-1960s retreat.
The Alps are highly impacted by debris flows that cause major problems for companies and transpor... more The Alps are highly impacted by debris flows that cause major problems for companies and transport networks located in the valley bottoms. One such event occurred in the Rif Blanc catchment and affected the road network in the French Alps, as well as adjacent areas across the Italian border, for several days in June 2012. This article presents two independent approaches to vulnerability assessment. Based on investigations conducted during a survey of local authorities following the event, we compared theoretical risk management and real crisis management in terms of decision making and modes of intervention. Functional vulnerability and territorial consequences were analyzed using a best travel time model of accessibility. We show that a bottom-up approach is practiced in case of actual management planning with a central coordination of general council. Conversely theoretical crisis management shows prefect as the key actor supported by several other state institutions. Our analysis also revealed that a debris flow event with a local impact on the road network has territorial consequences at a regional scale. This study contributes to the discussion about how to minimize the vulnerability of alpine transport networks prone to debris flows. Our results could serve as a decision support tool for public authorities.
The aim of this paper is firstly to characterise the debris flow frequency since the beginning of... more The aim of this paper is firstly to characterise the debris flow frequency since the beginning of the 20 th century in 6 valleys located on the eastern part of the Massif des Ecrins (French Alps). This analysis is based on the dating of 142 debris flow deposits by lichenometry, dendrochronology, aerial photographs analysis from 1952 and old documents. Results show that it is very difficult to compare the two periods of 1900-1950 and 1950-2000 because the number of debris flows over the first half of the century is probably underestimated. Over the second period , the frequency of debris flow has decreased significantly for the 200-399 m class lengths. We have also observed a decrease in the number of debris flows between 1950-1975 and 1975-2000. We have made two logistic regression models in order to determine an frequency probability according to geomorphological and climatic parameters. The first model shows that the frequency probability depends on the debris flow triggering zone altitude and the surface of the rock face, while the second model shows that the frequency probability depends on rainy events greater than 30 mm/day and also on the cumulated number of freezing days.
To assess the impacts of future climatic change, we need a better understanding of the relationsh... more To assess the impacts of future climatic change, we need a better understanding of the relationships between debris flows dynamic and the climate. The subject of this paper is the variability in the response of debris flows to climatic change in recent decades in the Massif des Ecrins (French Alps). The climatic trend observed in the French Alps was characterized by analyzing data on extreme summer rainfall events recorded daily at nine stations located all over the Massif des Ecrins since 1960. According to the generalized Pareto law (GPD) our results showed that extreme summer rainfalls have increased significantly in the Massif des Ecrins since the 1980s. Statistical tests showed a significant increase in average annual air temperature. The response of 647 hill slope debris flows to this climatic change was investigated. Different types of hill slope debris flows were analyzed as a function of their lithology or the nature of the accumulated debris. A number of logistic regression based models were developed to characterize the relationship between climate and the occurrence of debris flows in a specific geomorphological context. Analysis of frequency and return period over the last two decades showed different patterns depending on the type of debris flow concerned. Hence, the response of hill slope debris flows to climatic change is not the same everywhere in the Massif des Ecrins.
In this paper we investigate the impacts of future climatic change on the occurrence of debris fl... more In this paper we investigate the impacts of future climatic change on the occurrence of debris flows in the Massif des Ecrins (French Alps). Two distinct aspects are discussed: the impact of future climatic change on the evolution of the process, and changes in the spatial distribution. Three climate simulations are presented for the current period (1970–1999) and for future periods (2070–2099) using GCM ARPEGE CLIMAT model developed by Météo-France. Simulated data are then statistically downscaled to obtain a higher spatial resolution. In the first step, we compare occurrence probabilities in the current period and in the next century. In the second step, we estimate which zones would be affected by the process in the future at the scale of the Massif des Ecrins. For the current period, the best model was obtained between debris flows and the number of days between June 15th and October 15th with more than 20 mm calculated either from observed meteorological or simulated data. Results of the ARPEGE model considering the A2 hypothesis (IPCC 2007) showed that the most significant climatic trends for the end of the century will be a decrease in intense rainy events and an increase in temperature. These trends are expected to reduce the occurrence of hill slope debris flows in the Massif des Ecrins. From a spatial point of view, the increase in temperature should
Vincent, some of this paper is in British English and some in American, which changes the spellin... more Vincent, some of this paper is in British English and some in American, which changes the spelling modelling/modeling etc etc and you should have the same spelling throughout Abstract Reconstructing climate over the last millennium is one major target of climate research. The extent of climate variability over recent centuries sets the scale on which ongoing climate changes can be measured. The quality of reconstructions of millennial temperatures in the northern hemisphere has greatly improved over the last decade (IPCC, 2007). Nevertheless there is still a lack of precisely dated, high-quality climate reconstructions in the tropics, a lack which is particularly disturbing since this region is a major climate engine for the global water cycle. In recent years, a significant effort has been made to improve our knowledge on climatic variability in the tropical Andes by developing new proxy records, new transfer functions and better time resolution of available proxy records. Hence the aim of this paper is to review the different proxies with multi-decadal to annual resolution available in the tropical Andes (Bolivia, Peru, Ecuador) such as glacial moraines, pollens, isotopic variations in ice core records, and tree rings. For each proxy, we focus on recent major methodological progress and on new results revealing climatic shifts over the last millennium.
A total of 1500 spacing distances of sorted stripes were measured on 30 alpine sites in the Boliv... more A total of 1500 spacing distances of sorted stripes were measured on 30 alpine sites in the Bolivian Andes with distinct slope angles, elevations and aspects. Additionally, the geometric evolution of stripes was systematically analysed along a slope with varying inclinations and morphology. We dealt with mainly small-sized stripes (<40 cm). The scatter observed in the distribution of spacing distances suggests that the factors controlling the geometry of sorted stripes are complex. Among them, we point out the role of slope inclination and coarse sediment supply. They imply that stripe geometry does not depend exclusively on processes operating in soils such as frost heaving and sorting, but also on the dynamics of the flux of coarse materials along the sloping surface. This analysis questions models which assume that sorted stripes are organized according to a strict periodicity whose origin depends on specific physical processes. Particularly, it appears that recent convective models cannot explain the characteristics and the evolution of patterned slopes in this environment. R ´ ESUMÉ On a mesuré 1500 largeurs de stries réparties sur 30 sites se différenciant par la pente, l'altitude et l'exposition. Par ailleurs, on a analysé systématiquement l'´ evolution des stries le long d'un versant de pente et de morphologie variables. On confirme que dans ce type de milieu o` u dominent les cycles de gels-dégels journaliers les stries sont en majorité de petite dimension (<40 cm), mais la dispersion de la distribution suggère que les facteurs qui déterminent l'espacement des stries sont complexes. Parmi eux se détachent l'inclinaison de la pente et la morphologie du haut du versant, laquelle intervient par la quantité de produits clastiques parvenant au sol strié. Cela implique que la géométrie des sols striés ne dépende pas seulement des mécanismes du gel dans le sol, comme le gonflement et le tri qui lui est associé, mais aussi de la dynamique du flux de matériel grossier tout au long du versant. Cetté etude remet en question les modèles qui partent de l'hypothèse que les sols striés sont organisés selon une stricte périodicité, dont l'origine est attribuéè a un processus physique particulier. Il apparaˆıt notamment que les modèles construitsà partir de la convection ne parviennent pasà rendre compte des caractères et desévolutions observés sur les sols striés dans ce type de milieu.
SUMMARY Currently, there is a tremendous scientific research effort in the area of climate change... more SUMMARY Currently, there is a tremendous scientific research effort in the area of climate change. In this paper, our motivation is to improve the understanding of historical climatic events such as the Little Ice Age (LIA), a period of relatively cold weather around 1450–1850 AD. Although the LIA is well documented in Europe, its extent and timing are not known in areas of the globe where climatological records were not kept during this period. To study the climate, which predates historical records, proxy climate records must be used. A proxy record for the timing of climatic cooling events are the ages of the moraines left behind by glacial advances. Unfortunately, to determine the ages of these moraines in alpine environments there is little material available but lichens. Hence, lichenometry was developed to determine the ages of glacial landforms by using lichen measurements. To our knowledge, this article provides the first attempt at deriving a comprehensive statistical model for lichenometry. Our model foundation is based on extreme value theory because only the largest lichens are measured in lichenometry studies. This application is novel to extreme value theory because the quantities of interest (the ages of climatic events) are not the measured quantities (lichen diameters), i.e., it is a inverse problem. We model the lichen measurements with the generalized extreme value (GEV) distribution, upon which a Bayesian hierarchical model is built. The hierarchical model enables estimation of the hidden covariate ages of the moraines. The model also allows for pooling of data from different locations and evaluation of spatial differences in lichen growth. Parameter inference is obtained using a straightforward Markov Chain Monte Carlo method. Our procedure is applied to data gathered from the Cordillera Real region in Bolivia.
In this paper, we discuss the advantages and drawbacks of the most classical approaches used in l... more In this paper, we discuss the advantages and drawbacks of the most classical approaches used in lichenometry. In particular, we perform a detailed comparison among methods based on the statistical analysis of either the largest lichen diameters recorded on geomorphic features or the frequency of all lichens. To assess the performance of each method, a careful comparison design with well-defined criteria is proposed and applied to two distinct data sets. First, we study 350 tombstones. This represents an ideal test bed because tombstone dates are known and, therefore, the quality of the estimated lichen growth curve can be easily tested for the different techniques. Secondly, 37 moraines from two tropical glaciers are investigated. This analysis corresponds to our real case study. For both data sets, we apply our list of criteria that reflects precision, error measurements and their theoretical foundations when proposing estimated ages and their associated confidence intervals. From this comparison, it clearly appears that two methods, the mean of the n largest lichen diameters and the recent Bayesian method based on extreme value theory, offer the most reliable estimates of moraine and tombstones dates. Concerning the spread of the error, the latter approach provides the smallest uncertainty and it is the only one that takes advantage of the statistical nature of the observations by fitting an extreme value distribution to the largest diameters.
The Younger Dryas stadial, a cold event spanning 12,800 to 11,500 years ago, during the last degl... more The Younger Dryas stadial, a cold event spanning 12,800 to 11,500 years ago, during the last deglaciation, is thought to coincide with the last major glacial re-advance in the tropical Andes1. This interpretation relies mainly on cosmic-ray exposure dating of glacial deposits. Recent studies, however, have established new production rates2–4 for cosmogenic 10Be and 3He, whichmake it necessary to update all chronologies in this region1,5–15 and revise our understanding of cryospheric responses to climate variability. Here we present a new 10Be moraine chronology in Colombia showing that glaciers in the northern tropical Andes expanded to a larger extent during the Antarctic cold reversal (14,500 to 12,900 years ago) than during the YoungerDryas.On the basis of a homogenized chronology of all 10Be and 3He moraine ages across the tropical Andes, we show that this behaviour was common to the northernand southern tropical Andes. Transient simulations with a coupled global climate model suggest that the common glacier behaviour was the result of Atlanticmeridional overturning circulation variability superimposed on a deglacial increase in the atmospheric carbon dioxide concentration. During the Antarctic cold reversal, glaciers advanced primarily in response to cold sea surface temperatures over much of the Southern Hemisphere. During theYoungerDryas, however,northern tropicalAndes glaciers retreated owing to abrupt regional warming in response to reduced precipitation and land–surface feedbacks triggered by a weakened Atlantic meridional overturning circulation. Conversely, glacier retreat during the Younger Dryas in the southern tropical Andes occurred as a result of progressive warming, probably influenced by an increase in atmospheric carbon dioxide. Considered with evidence frommid-latitude Andean glaciers16, our results argue for a common glacier response to cold conditions in the Antarctic cold reversal exceeding that of the Younger Dryas.
Much work has been done to show that there is a relationship between the triggering of debris flo... more Much work has been done to show that there is a relationship between the triggering of debris flows and the recorded increase in temperatures or in the number of intense rainy events over the last few decades. The question addressed in this paper is that of the impact of these climate changes on the dynamics of debris flows since the 1950s. 319 debris flows in the Dévoluy and Ecrins massifs located in the French Alps, the triggering of which is independent of the current glacial retreat, have been analysed. In the Dévoluy a reduction in the number of debris flows was observed in the periods 1950–1975 and 1975–2000. In the Massif des Ecrins, we have observed a shift of the triggering debris flow zone toward higher elevations and a lack of significant variation in the number of debris flows. But in the Massif des Ecrins this global result masks two different trends depending on elevation. At low altitude (<2200 m) the number of debris flows and the frequency of debris flows less than 400 m in length have decreased significantly since the 1980s whereas no significant variation was observed at high altitude (>2200 m). At the same time, we have observed a significant increase in the annual and seasonal temperatures for these 20 last years combined with a significant reduction in the number of freezing days. A significant increase in summer rains higher than 30 mm/d has also been observed. In Dévoluy and at low altitude in the Massif des Ecrins, these variations can be explained by the decrease in the number of freezing days related to the increase in the temperatures, which implies a slower reconstitution of the volume of debris stored between two events. But at high altitude it is currently difficult to establish the link between the climatic change and the dynamics of the debris flows because very little is known about the two variables controlling the triggering of the debris flows, i.e., on the one hand intense precipitations and on the other hand the volume of rock debris.
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Papers by V. Jomelli
and 3He moraine ages across the tropical Andes, we show that this behaviour was common to the northernand southern tropical Andes. Transient simulations with a coupled global climate model suggest that the common glacier behaviour was the result of Atlanticmeridional
overturning circulation variability superimposed on a deglacial increase in the atmospheric carbon dioxide concentration. During the Antarctic cold reversal, glaciers advanced primarily in response to cold sea surface temperatures over much of the Southern Hemisphere.
During theYoungerDryas, however,northern tropicalAndes
glaciers retreated owing to abrupt regional warming in response to reduced precipitation and land–surface feedbacks triggered by a weakened Atlantic meridional overturning circulation.
Conversely, glacier retreat during the Younger Dryas in the southern tropical Andes occurred as a result of progressive warming, probably influenced by an increase in atmospheric carbon dioxide. Considered with evidence frommid-latitude Andean glaciers16, our results argue for a common
glacier response to cold conditions in the Antarctic cold reversal exceeding that of the Younger Dryas.
and 3He moraine ages across the tropical Andes, we show that this behaviour was common to the northernand southern tropical Andes. Transient simulations with a coupled global climate model suggest that the common glacier behaviour was the result of Atlanticmeridional
overturning circulation variability superimposed on a deglacial increase in the atmospheric carbon dioxide concentration. During the Antarctic cold reversal, glaciers advanced primarily in response to cold sea surface temperatures over much of the Southern Hemisphere.
During theYoungerDryas, however,northern tropicalAndes
glaciers retreated owing to abrupt regional warming in response to reduced precipitation and land–surface feedbacks triggered by a weakened Atlantic meridional overturning circulation.
Conversely, glacier retreat during the Younger Dryas in the southern tropical Andes occurred as a result of progressive warming, probably influenced by an increase in atmospheric carbon dioxide. Considered with evidence frommid-latitude Andean glaciers16, our results argue for a common
glacier response to cold conditions in the Antarctic cold reversal exceeding that of the Younger Dryas.